High temperatures can be achieved with the help of resistance furnaces. Electrical current flows through an ohmic resistor designed as a heat conductor, the electrical power being mainly converted into heat. Metal, such as molybdenum, tantalum, and platinum, ceramics, SiC, or modifications of carbon, such as coal, graphite or vitreous carbon (pyrolytically produced carbon) are suited as a material for the heat conductor. Heat conductors of graphite are characterized by their high temperature resistance, simple shape and low price.
Resistance furnaces are e.g. used for melting semiconductor material, or for heating rod-like or tubular start cylinders to elongate tubes, rods or optical fibers therefrom. Apart from special cases in which a purposefully inhomogeneous heating capacity is desired (e.g. during drawing of cylinders having a polygonal cross-section), the main emphasis is normally laid on a uniform heating of the heating material.
The local heating capacity is directly proportional to the current density, the latter being defined by the current flow and the cross-sectional area and the specific resistance of the material of the heat conductor. The specific resistance, in turn, depends on the local temperature. The relatively low conductivity of the heat conductor materials and the accompanying voltage drop make it difficult to produce and maintain a homogeneous temperature profile.
U.S. Pat. No. 4,703,556 therefore suggests a furnace having a heating tube of graphite, in which a plurality of axially extending longitudinal slots are distributed over the circumference of the heating tube and extend in alternating fashion from above and from below over almost the whole heating tube height. Thus, electricity flows through the remaining webs of the heating tube in meander-like fashion. This results in a homogenization of the temperature curve in vertical direction.
A further homogenization of the current density within the heating tube is accomplished by the measure that two graphite connection pieces are provided for the supply of heating current, said pieces being screwed in the area of the bottom side of the heating tube to opposite places and being fed via separate transformers. The voltage drop can be counteracted by the second power supply point over the entire length of the heating tube webs, resulting in an improved vertical and horizontal homogeneity of the current and temperature distribution.
It is however very complicated to produce the known heating tube. Due to its filigree shape it is prone to mechanical damage and must therefore be replaced frequently. Moreover, due to scaling and oxidation in the region of the supply terminals there might be a deterioration of the electrical contact and an undefined power supply and thus an irregular heating operation and temperature distribution over the heating tube. This risk is enhanced by the fact that the supply terminals are located directly next to the zone with the maximum temperature load.
In high-performance resistance furnaces (above about 100 kW), temperature sinks are particularly noticed and make it more and more difficult to adjust and maintain a homogeneous temperature distribution.